Fair Value Measurements - Assets and Liabilities Carried at Fair Value and Measured on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Total assets	$ 15,570	$ 15,301
Money Market Funds [Member]
Assets:
Total assets	15,070	7,155
Short-Term Investments [Member]
Assets:
Total assets	500	7,650
Long-term Investments [Member]
Assets:
Total assets		496
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets:
Total assets	15,070	7,155
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money Market Funds [Member]
Assets:
Total assets	15,070	7,155
Significant Other Observable Inputs (Level 2) [Member]
Assets:
Total assets	500	8,146
Significant Other Observable Inputs (Level 2) [Member] | Short-Term Investments [Member]
Assets:
Total assets	$ 500	7,650
Significant Other Observable Inputs (Level 2) [Member] | Long-term Investments [Member]
Assets:
Total assets		$ 496